Question

If the first terms of an A.P is 7 & 13 term is 35. Find the sum of first 13 terms.​

Answer 1

given

a=7

a¹³ = 35 it means no of terms = 13

so n = 13.

we know

aⁿ = a+(n-1)d

so put the values .----

35 = 7 +(13-1)d

28 = 12d

so

d = 28/12 => 7/3

Now sⁿ = n/2(a+aⁿ)

hence we get

sⁿ = 13/2(7+35)

=> 13/2(42)

=>21×13

=> 273

Answer 2

Answer:

273

Step-by-step explanation:

Sum of an AP can be given by $S_{n} = n/2( a + last)$

n is 13, a is 7 and last term is 35

So, 7+35 is 42, and half of that is 21.

n times 21 is left, n is 13.

273 is the result